In recent years, the chemistry pertaining to the selective oxidation of olefins was dominated by OsO.sub.4 and O.sub.3 Os.dbd.N--X species, the essential reactants in the catalytic asymmetric dihydroxylation (AD) and aminohydroxylation (AA) processes, respectively (Kolb et al. Chem. Rev. 1994, 94, 2483; Schlingloff et al. in Asymmetric Oxidation Reactions: A Practical Approach, Katsuki, T., Ed.: Oxford University Press, in press). Our continuing search for new transition metal-catalyzed heteroatom transfer reactions has centered around osmium's neighbors in the Periodic Table. Among the corresponding high valent oxo derivatives, methylrhenium trioxide (CH.sub.3 ReO.sub.3 or MTO) has been known for a long time (Beattie et al. Inorg. Chem. 1979, 18, 2318). It was only recently, however, that Herrmann and others developed MTO into a well defined catalyst for a variety of processes including olefin epoxidation with aqueous hydrogen peroxide (H.sub.2 O.sub.2). For applications of MTO in organic synthesis, see: Hoechst AG (Herrmann et al. ) DE 3.902.357 (1989); Herrmann et al. Angew. Chem., Int. Ed. Engl. 1991, 30, 1638; Herrmann et al. J. Mol. Catal. 1994, 86, 243; Herrmann et al. Organomet. Chem. 1995, 500, 149; Al-Ajlouni et al. Am. Chem. Soc. 1995, 117, 9243; Pestovsky et al. J. Chem. Soc., Dalton Trans. 2 1995, 133; Adam et al. Angew. Chem. Int. Ed. 1996, 35, 533; Boelow et al. Tetrahedron Lett. 1996, 37, 2717; Al-Ajlouni et al. J. Org. Chem. 1996, 61, 3969; Herrmann et al. J. Mol. Cat. 1997, 118, 33; Herrmann et al. Acc. Chem. Res. 1997, 30, 169; Espenson et al. Adv. Chem. Ser. 1997, 253, 99; ARCO Chemical Technology (Crocco et al., H. S.) U.S. Pat. No. 5,166,372 (1992).
Regarding olefin oxidation, there is a fundamental. difference between OsO4 and CH.sub.3 ReO.sub.3, for in contrast to OsO.sub.4, MTO does not react directly with olefins (This is true regarding olefin epoxidation. However, MTO is known to exhibit metathesis activity (Herrmann et al. Acc. Chem. Res. 1997, 30, 169). Rather, the MTO-catalyzed epoxidation is believed to proceed through the initial activation of H.sub.2 O.sub.2 by the electrophilic Re(VII) center resulting in the formation of equilibrating mixture of mono- and bisperoxorhenium complexes that transfer oxygen atoms to the corresponding olefins. Notably, the OsO.sub.4 /H.sub.2 O.sub.2 system has little synthetic value for olefin epoxidation. Even though epoxides are the primary products in this system, significant amounts of diols and overoxidation products are formed (Milas et al. J. Am. Chem. Soc. 1936, 58, 1302).
The major limitation of Herrmann's original MTO/H.sub.2 O.sub.2 epoxidation system is the acidity of the reaction medium. The water molecule coordinated to the Re(VII) center of the bisperoxo complex is highly acidic and sensitive epoxides do not survive (The water molecule coordinated to the rhenium center of the bisperoxo complex of MTO is highly acidic: Herrmann et al. Angew. Chem. Int. Ed. Eng. 1993, 103, 1991). Recent efforts in our laboratory led to a highly efficient olefin epoxidation with 30% aqueous H.sub.2 O.sub.2 where the catalytic activity of MTO was uncoupled from acidity for the first time (Rudolph et al. J. Am. Chem. Soc. 1997, 119, 6189; Coperet et al. Chem. Commun. 1997, 16, 1565). The crucial features of this new process are the requirement for a pyridine ligand and the solvent switch from tert-butyl alcohol to methylene chloride which additionally enhances the effectiveness of the pyridine-modified rhenium catalyst (FIG. 1A).
We have previously disclosed on further improvements in this epoxidation catalysis, specifically on the use of 3-cyanopyridine as a ligand of choice for the epoxidation of terminal and trans-disubstituted olefins (Coperet et al. Chem. Commun. 1997, 16, 1565).
What is needed is an efficient and improved method for oxorhenium epoxidization of diversely functionalized olefins wherein the improvement increases turnover and which subsequently reduces diol side products obtained from epoxide ring opening and increases the yield of the desired epoxide product.